1. Field of the Invention
The present invention relates to a gas sensor, such as an oxygen sensor, an HC sensor, or an NOx sensor, for detecting a component of a gas to be measured.
2. Description of the Related Art
A known gas sensor of the type described above has a rodlike or cylindrical detection element having a detection portion for detecting a component of interest. The detection portion is formed at the front end of the detection element, which is housed in a metallic casing. This gas sensor has a protector that covers the detection portion placed within an environment to be measured. Gas passage holes are formed in the sidewall of the protector. A gas such as exhaust gas to be measured is guided into the protector via the gas passage holes and brought into contact with the detection portion.
Many of various gas sensors recently developed for use in automobiles use a double-structure protector consisting of inner and outer cylindrical portions to provide enhanced protection for the detection portion from water droplets, oil droplets, and contaminants. The conventional double-structure protector indicated by 106 in FIG. 9 has the inner and outer cylindrical portions 106a and 106b. Gas inlets 163 and 161 are formed in the sidewalls of the cylindrical portions, respectively. Gas under measurement first passes through the gas inlets 163 in the outer cylindrical portion 106a and then passes through the gas inlets 161 in the inner cylindrical portion 106b to the detection portion 102.
In the protector of the double structure described above, the protecting performance of the detection portion is enhanced. However, the resistance to the passing gag is increased because of the double wall structure. For example, the rate at which the gas under measurement is exchanged is often low in the space between the outside and inside of the protector. Therefore, the structure has the problem that the response tends to be delayed if the concentration of the component under measurement within the measured ambient varies rapidly.
If the detection portion 102 shown in FIG. 9 has a gas-sensitive surface DP on only one side of a lamination, the following problem takes place. If gas EG to be measured such as exhaust gas flows into the protector 106 from the side of the gas-sensitive surface DP, the gas stream arrives at the gas-sensitive surface DP relatively directly. Therefore, where the concentration of the component under measurement changes, the detection response is relatively good. However, where the gas flows from the opposite side, the gas stream strikes the surface of the detection portion 102 on the opposite side of the sensitive surface DP. This tends to delay the response of the detection. In this way, the sensor has the drawback that the response and output characteristics are easily affected according to the direction of the gas stream to be measured relative to the protector.
If the protector is built as a single structure, the rate at which the gas is exchanged between the inside and outside of the protector is enhanced so that the response of the sensor is improved. Of course, however, the function of protecting the detection portion deteriorates. If the gas flow rate increases rapidly, or if the gas temperature drops quickly, the temperature of the detection portion drops, thus deactivating an oxygen concentration cell (also known as a differential aeration cell). This leads to a deterioration of the detection sensitivity, or the output signal from the cell is interrupted. The gas exchange rate may also be increased by increasing the dimensions of the gas inlets in the double-structure protector. This structure inevitably produces problems similar to those with the aforementioned single-structure protector, though the results may be more or less serious. Hence, it has been difficult to achieve good response and high protecting performance simultaneously.
It is therefore an object of the present invention to provide a gas sensor which is provided with a multiple-structure protector and thus protects a detection element well and which has sensor response characteristics depending to a lesser extent on the direction of the stream of a gas under measurement than conventional and, therefore, produces uniform response or output characteristics at an appropriate level.
The above object of the present invention has been achieved by providing a gas sensor which comprises a detection element having a front-end portion, a detection portion formed on or in the front-end portion of the detection element, and a protector that covers the detection portion. The protector comprises a first cylindrical portion and a second cylindrical portion disposed outside said first cylindrical portion. A tapering reduced portion is formed at an axially front-end side of a sidewall portion of said first cylindrical portion. A second side gas inlet is formed in a sidewall portion of said second cylindrical portion and corresponds in position to said reduced portion.
In the above-described gas sensor in accordance with the present invention, the protector is of the multiple structure, i.e., at least the double structure having the inner first cylindrical portion and the outer second cylindrical portion. This suppresses intrusion of water droplets, oil droplets, and so on into the protector. Hence, the instrument is excellent in protecting the detection portion.
Furthermore, a tapering reduced portion is formed at an axially front side of the sidewall portion of the first cylindrical portion. Second side gas inlets are formed in the sidewall portion of the second cylindrical portion and correspond in position to the reduced portion. Gas stream to be measured strikes the reduced portion and flows along the outer surface of the sidewall portion of the reduced portion. This produces a negative pressure at the side of the first side gas outlet, decreasing the pressure inside the first cylindrical portion. The gas under measurement is quickly drawn in from the first gas inlets spaced circumferentially. In consequence, sufficient response can be secured in spite of the multiple structure of the protector.
Where the first cylindrical portion is cut along a plane including the axis, the cross section through the tapering portion may be straight or curved outwardly or inwardly. Setting the diameter of the front end of the reduced portion smaller than that of the base end is preferable for production of a negative pressure in the first side gas outlet.
More specifically, the first cylindrical portion has the tapering portion that is a truncated cone coupled to the front end of a cylindrical body. For example, once the total length of the first cylindrical portion is set, the tilt angle of the outer surface of the tapering portion can be easily adjusted to a value adapted for production of a negative pressure in the first side gas outlet by adjusting the length of the cylindrical body formed at the side of the base end.
Accordingly, a specific embodiment of the present invention comprises:
a detection element for detecting a detected constituent of a gas under measurement by a detection portion formed in a front-end portion;
a cylindrical element container that covers the detection element while permitting the detection portion to protrude; and
a protector coupled to an opening end portion in the element container through which the detection portion protrudes, the protector covering the detection portion while permitting passage of the gas to be measured;
said protector comprising
a first cylindrical portion circumferentially surrounding the detection portion around the axis of the detection element, the first cylindrical portion having a sidewall portion provided with plural first side gas inlets circumferentially spaced from each other at regular intervals, a tapering reduced portion being formed at an axially front end side of the sidewall portion, the reduced portion having a front-end surface provided with a first gas outlet, and
a second cylindrical portion having an opening portion at its front end and located outside the first cylindrical portion such that a given gap is formed between the first and second cylindrical portions, the second cylindrical portion permitting the gas under measurement to flow along the tapering outer surface of the reduced portion from its base end toward the front end and inhibiting the gas under measurement from directly flowing into the first side gas inlets in a direction perpendicular to the axis of the first cylindrical portion where the second cylindrical portion is placed within the gas stream under measurement, said reduced portion of said first cylindrical portion protruding from the opening portion in the second cylindrical portion in such a way that the first side gas outlet is located closer to the base end than the front-end surface of the second cylindrical portion in the direction of the axis or that inner fringes of the opening portion in the second cylindrical portion are located opposite to the outer surface of the reduced portion.
The detection portion described above can be fabricated as an oxygen detection portion comprising an oxygen concentration cell and a planar heater. The oxygen concentration cell is built by forming a first porous electrode at one side of a board-like layer of an oxygen ion conductive solid electrolyte and a second porous electrode at the opposite side. The first porous electrode forms a detection electrode. The second electrode forms a reference oxygen electrode. The planar heater is formed on the reference oxygen-side porous electrode. The electrode surface of the detection-side porous electrode forms a gas-sensitive surface. This structure can be preferably used in a xcex-type oxygen sensor, for example.
The above-described gas sensor in accordance with the present invention has the first cylindrical portion having the sidewall portion provided with plural first side gas inlets circumferentially spaced from each other at regular intervals. The first side gas inlets are formed by two sets of holes axially spaced from each other. The holes of one of the two sets are formed so as to be located opposite to the detection portion. The holes of the other set are located closer to the front end than the front end of the detection portion. For example, if the first side gas inlets consist of only one array of holes corresponding to the detection portion, and if a negative pressure increases the rate at which the gas under measurement flows into the first cylindrical portion, a major part of the entering gas will strike the detection portion. If the gas contains water droplets or the like, the water droplets strike the detection portion, impairing the protecting function of the protector. Accordingly, if one or more arrays of holes are added ahead of the front end of the detection element, the flow of the water droplets is dispersed and, therefore, the protecting function can be maintained.
The second cylindrical portion permits the gas under measurement to flow along the outer surface of the tapering portion at the front end of the first cylindrical portion from its base end toward the front end. This stream of gas develops a negative pressure in the first side gas outlet formed at the front end of the tapering portion. Thus, the interior of the first cylindrical portion is evacuated. Consequently, the gas under measurement is drawn into the first cylindrical portion almost isotropically from the peripherally spaced first gas inlets. As a result, uniform response or output can be obtained, irrespective of the angle at which the gas stream strikes the surface of the detection portion from around the axis of the protector, i.e., regardless of the direction of the gas stream. This is especially advantageous where a gas-sensitive surface is formed in a part of the peripheral outer surface of the detection portion or where a gas-sensitive surface in the form of a planar, thin or thick film is formed at one side.
Another gas sensor in accordance with the present invention has the first cylindrical portion having a front-end surface provided with a first side gas outlet. A second side gas outlet is formed in a front-end surface of the second cylindrical portion. The second side gas outlet is located closer to the front end than the first side gas outlet. In this configuration, the measured gas introduced from the second side gas inlets directly collides against the tapering portion, increasing the flow velocity of the gas along the tapering portion. Hence, the negative pressure produced in the first side gas outlet can be increased. As a result, the rate at which the measured gas is drawn in from the first side gas inlets is increased. Thus, the rate at which the measured gas is exchanged within the first cylindrical portion is increased. Consequently, the detection response or the response of the output to concentration variations is improved further.
To make effective use of the sucking effect described above, the second side gas outlet in the gas sensor in accordance with the present invention can be made larger than the first side gas outlet. In this configuration, outflow of the gas under measurement from the second side gas outlet is not hindered and thus smooth. Variations in the negative pressure produced in the first side gas outlet can be suppressed. Consequently, stable detection response or output response to concentration variations can be obtained.
The first and second side gas outlets can be formed coaxially with each other. In this configuration, the measured gas drawn in from the first cylindrical portion via the first side gas outlets can be smoothly expelled from the second side gas outlets, together with the gas flowing along the tapering portion. This can further improve the detection response of the sensor or the output response to variations in the concentration.
A further gas sensor in accordance with the present invention has the first cylindrical portion having a front-end surface provided with the first side gas outlet and the second cylindrical portion having a front-end surface provided with the second side gas outlet. The first side gas outlet can be located closer to the front-end side than the second side gas outlet. In this configuration, the second side gas outlet directly faces the reduced portion of the front-end side of the first cylindrical portion. If the gap between the outer surface of the reduced portion and the inner fringes of the second side gas outlet is narrowed, the flow velocity of the gas under measurement along the reduced portion is increased further. The negative pressure produced in the first side gas outlet can be increased further. Accordingly, this further increases the rate at which the gas under measurement inside the first cylindrical portion is exchanged. The detection response or the output response to concentration variations can be improved further.
In still another gas sensor in accordance with the present invention, the second side gas inlets are formed as follows. Breaks are formed in the sidewall portion of the second cylindrical portion such that the breaks have direction change portions that change direction from one base end portion and go to the other base end portion. Claw-like portions surrounded by the breaks are bent radially inwardly, thus forming the second side gas inlets. In this configuration, the claw-like portions overlap the second side gas inlets like flaps. This suppresses intrusion of water droplets, oil droplets, and so on into the protector. This further improves the protection of the detection portion. Furthermore, the flow of the gas under measurement is made to circulate along the sidewall portion of the second cylindrical portion by the claw-like portions bent radially inwardly. In consequence, the flow velocity of the gas along the reduced portion is increased. The negative pressure produced in the first side gas outlet can be increased further. As a result, the rate at which the gas under measurement inside the first cylindrical portion is exchanged is enhanced. Hence, the detection response or output response to concentration variations can be improved further.
In yet another gas sensor in accordance with the present invention, the holes of the set located opposite to the detection portion form breaks in the sidewall portion of the first cylindrical portion. The breaks have direction change portions that change direction from one base end portion and go to the other base end portion. Claw-like portions surrounded by the breaks are bent radially inwardly. In this structure, the claw-like portions overlap the first side gas inlets like flaps. This suppresses intrusion of water droplets, oil droplets, and so on into the first cylindrical portion. This enhances the protection of the detection portion. Furthermore, the flow of the gas under measurement is made to circulate along the sidewall portion of the first cylindrical portion by the claw-like portions bent radially inwardly. The gas is sucked almost isotropically. As a result, the gas under measurement makes uniform circulating flow around the axis of the protector. Uniform response or output can be obtained.
Other objects and features of the invention will appear in the course of the description thereof, which follows.